NITROGENOUS METABOLISM 149 



their power to minister to growth and repair. Among 

 an increasing number of defective proteins now recognized 

 gelatin is the best known. Its behavior in the system calls 

 for exposition. 



Gelatin. This familiar compound belongs to the ill- 

 defined group often spoken of as the albuminoids. These 

 may fairly be regarded as proteins which have undergone 

 a more or less definite degeneration both in function and 

 in chemical structure. They are found as intercellular 

 substance in the connective tissues and also in the dead 

 and dry surface layer of the skin. They make the chief 

 substance of the hair and nails. Gelatin itself is derived 

 by boiling certain varieties of connective tissue, including 

 bone and tendon. It gives nearly average percentages of 

 the five elements present in ordinary proteins. When this 

 fact was discovered in the early days of organic chemistry 

 it was urged that gelatin must have a value in nutrition 

 quite equal to that of any nitrogenous food. 



Trials were made on a large scale in the pauper institu- 

 tions of France. It was found very definitely that the 

 free use of gelatin led to indigestion and that it soon 

 became repugnant to the subjects, however hungry they 

 might be. These effects were later found to be correlated 

 with inadequacy to maintain the weight and strength of 

 animals. An animal cannot be said to be perfectly nour- 

 ished if it is losing more of any element day by day than it 

 is receiving. This is eminently true of the nitrogen in the 

 income and the outgo. "Nitrogenous equilibrium," 

 expressing the equality of the two, is a phrase we shall use 

 freely. Now nitrogenous equilibrium cannot be estab- 

 lished by feeding gelatin in place of all other protein, no 

 matter how skilfully the experiment is conducted. 



For many years the insufficiency of gelatin to make good 

 the losses from the tissues remained a mystery. It has 

 been amply explained by the findings of chemists in our 

 own time. Gelatin yields most of the building-stones re- 

 quired for the new construction, but it does not furnish 

 them all. Therefore it is impossible to make blood-pro- 



